Intersection of Two Linked Lists

07/15/2016 Linked List

Question

Write a program to find the node at which the intersection of two singly linked lists begins.

For example, the following two linked lists:

     A:          a1 → a2
                        ↘
                          c1 → c2 → c3
                        ↗            
     B:     b1 → b2 → b3

begin to intersect at node c1.

Notes:

  • If the two linked lists have no intersection at all, return null.
  • The linked lists must retain their original structure after the function returns.
  • You may assume there are no cycles anywhere in the entire linked structure.
  • Your code should preferably run in O(n) time and use only O(1) memory.

Solution

Result: Accepted Time: 36 ms

Here should be some explanations.

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     struct ListNode *next;
 * };
 */
struct ListNode *getIntersectionNode(struct ListNode *headA, struct ListNode *headB) {
    struct ListNode * a = headA,*b = headB,*tmp;
    int cnt = 0;
    while(a && b)
    {
        a = a->next;
        b = b->next;
        cnt ++;
    }
    if(a == NULL)
    {
        tmp = headB;
        while(b)
            tmp = tmp->next,
            b = b->next;
        b = tmp;
        a = headA;
    }
    if(b == NULL)
    {
        tmp = headA;
        while(a)
            a = a->next,
            tmp = tmp->next;
        a = tmp;
        b = headB;
    }
    while(a != b && a && b)
    {
        a = a->next;
        b = b->next;
    }
    if(a && b) return a;
    return NULL;
}

Complexity Analytics

  • Time Complexity:
  • Space Complexity: